Cdh1, a co-activator of the Anaphase Promoting Complex (APC) has recently been shown to be important in germinal vesicle stage arrest and in the prevention of aneuploidy during the first meiotic division of mouse oocytes. However, this was through antisense knockdown approaches done in-vitro. Therefore, here I generated an oocyte-specific knockout of Cdh1 to explore this further. In this way, Cdh1 protein was specifically deleted only in germinal vesicle (GV) stage oocytes from growing follicles. Fewer cumulus enclosed oocytes were observed from Cdh1 knockout mice, of which significant numbers had undergone GVB. Furthermore, significantly more meiotically advanced, fragmented or degenerate oocytes were observed in knockouts. Denuded Cdh1 knockout GV oocytes also displayed a propensity for spontaneous GV breakdown (GVB) which could be partially rescued by maintaining an intact cumulus mass. Cdh1 knockout oocytes also underwent accelerated GVB on release from arresting media. Western Blots revealed a 5 fold increase in cyclin B1 levels following loss of Cdh1, whereas other substrates of APCCdh1, securin and Cdc25B, remained unchanged. In-vivo and in-vitro matured metaphase II (MII) oocytes were analysed for aneuploidy rates. In-vivo matured knockout oocytes had higher, but not statistically significant, rates of aneuploidy than controls. Denuded oocytes that underwent IVM also had a higher incidence of aneuploidy in knockouts and in this group this was highly significant. In summary, data from the in-vivo knockout model supports those of the in-vitro antisense approach and provide further evidence for the role of Cdh1 in both GV arrest and the prevention of aneuploidy, at least in the situation where oocytes are cultured in-vitro.